1. Field of the Invention
The present invention relates to automatic gain control in a zero intermediate frequency radio device such as a receiver or transceiver. Such a radio device processes down-converted and demodulated received radio frequency signals, and, when a transmit part is also present, transmits modulated and up-converted signals. Such radio devices can be cellular radio, cordless telephony, or, wireless local area network radio devices, satellite radio devices, or any other suitable radio device.
2. Description of the Related Art
From the handbook xe2x80x9cRF and Microwave Circuit Design for Wireless Communicationsxe2x80x9d, L. E. Larson, Artech House Publishers, 1996, page 73, a Direct-Conversion zero-IF receiver is known. In such a receiver, a local oscillator comprised in the receiver is tuned to a carrier frequency of the incoming radio frequency signal. When DC-coupling stages in such a direct conversion zero-IF receiver, serious problems are caused by DC-offset such as due to LO-leakage to an input of a low noise radio frequency amplifier that is usually present between an antenna and a mixer of the zero-IF radio device, and further due to DC-offset in various components of the radio device, such as in channel filters, amplifiers, or in other components. To mitigate such DC-offset problems, AC-coupling is provided in the receive branch of the radio device. Such an AC-coupling can be distributed over various stages whereby all stages are designed such that the DC-offset of a stage is much smaller than the dynamic range of that stage.
In the U.S. Pat. No. 5,982,807, an intermediate frequency spread spectrum radio transceiver is disclosed for use in wireless local area network, in the so-called 2.4 GHz ISM band as defined in the IEEE 802.11b standard. In the transceiver, a baseband processor comprises a demodulator for spread spectrum phase shift keying (PSK) demodulating information received from a radio circuit comprised in the transceiver. In addition to a bi-phase or binary PSK mode (BPSK), the transceiver can operate in a quadrature PSK mode (QPSK). The demodulator is connected to an output of an analog-to-digital converter. The analog-to-digital converter is AC-coupled to the radio circuit. For substantially reducing an average DC-component, a particular type of Walsh code is used. As shown in FIG. 1 of U.S. Pat. No. 5,982,807, the wireless transceiver has an antenna, an up/down converter, and a Tx/Rx-switch. The up/down converter is connected to a low noise radio frequency amplifier in a receive branch of the transceiver, and to a radio frequency power amplifier in a transmit branch of the transceiver. The up/down converter is connected to a frequency synthesizer and to an IF modulator/demodulator. The transceiver further comprises various filters, and voltage controlled oscillator. A baseband processor comprise high speed 3-bit analog to digital converters for receiving the quadrature I and Q signals from the modulator/demodulator. Furthermore, the baseband processor includes a received signal strength indicator monitoring function with a 6-bit analog to digital converter.
On page 62 of the DRAFT Supplement, Part 11, to the above IEEE 802.11b standard, operating channels are shown for North American Channel Selection. With a local oscillator in the radio tuned to 2412 MHz, the zero-IF radio device receives radio signals from the shown non-overlapping Channel 1.
In the U.S. Pat. No. 5,982,235, an automatic gain control circuit (AGC) is disclosed which is used for mobile communication. As shown in FIG. 7 of U.S. Pat. No. 5,982,235, the gain of an amplifier is set. The amplifier that amplifies an input IF signal, has a gain control function. The thus-amplified signal is output to a demodulation circuit. For application of an AGC in mobile communication, as described, a receiving level changes as great as +10 dB or more to xe2x88x9230 dB or less. In order to take care of a significant drop in the receiving level in excess of the range of control of the AGC, such as due to a fading phenomenon, the shown automatic gain control circuit comprises a fading detection circuit at IF (RSSI), an AGC convergence level setting circuit, a signal-to-noise (S/N) detection circuit, and an AGC setting circuit. The S/N detection circuit that is connected to an output of the amplifier provides one input signal to the AGC convergence level setting circuit. The RSSI provides another input signal to the AGC convergence level setting circuit. The AGC circuit further comprises an attenuation setting circuit coupled to an output of the amplifier. The output signal of the AGC circuit occurs at an output of the attenuation setting circuit. The RSSI detects whether the AGC circuit is on the move at high speed. If this is the case, the AGC convergence level setting circuit controls the AGC convergence level so as to increase or decrease, thereby preventing loss of data. If this is not the case, the ratio of an output signal to noise and the output signal level are maintained at constant levels. If a fading occurs, the level of AGC convergence is increased thereby preventing deterioration of the ratio of the output signal to noise. The attenuation circuit is set such that the level of the output signal remains constant.
It is an object of the invention to provide an automatic gain controller in a zero intermediate frequency radio device with AC-coupled stages, whereby a signal resolving range of a received signal strength indicator is below a high dynamic range exhibited by an incoming radio frequency signal.
It is another object of the invention to provide such an automatic gain controller that stepwise iterates to providing an output signal to be sampled in a linear range of the received signal strength indicator, either by initially starting with a maximum gain or with a minimum gain.
It is still another object of the invention to provide such an automatic gain controller that first kicks off by modifying the gain of the low noise radio frequency amplifier (LNA) where the largest DC-offset problems exist and where the effect of out-of-band jammers can be reduced by decreasing the gain of the LNA.
It is still another object of the invention to reduce the negative effects of AC-coupling after the AGC has settled, by reducing the cut-off frequency of the AC-coupling.
It is still another object of the invention to distribute gain control over components of the receive branch between the antenna and the signal processor for processing the zero-IF signal.
In accordance with the invention, a zero intermediate frequency radio device is provided comprising:
an antenna for receiving a radio frequency signal, said radio frequency signal exhibiting a high dynamic range;
a frequency down converter for down converting said radio frequency signal to a zero intermediate frequency signal, said frequency down converter comprising a mixer, an AC-coupler, and a received signal strength indicator with a signal resolving range that is below said high dynamic range, said AC-coupler being coupled to an output of said mixer;
a signal processor for processing said zero intermediate frequency signal;
at least one amplifier coupled between said antenna and said signal processor; and
an automatic gain controller for at least gain controlling said at least one amplifier,
said automatic gain controller being configured to set a gain of said at least one amplifier by setting said gain to a predetermined gain, by waiting a predetermined time for allowing DC-offset signals in said radio device to decay, by checking whether a reading of said received signal strength indicator is within said signal resolving range, and by setting said gain in accordance with said reading if said reading is within said signal resolving range.
The invention is based upon the insight that in zero intermediate frequency radio device with AC-couplers, the AGC can only be set if no signal saturation at the output of the RSSI occurs, due to DC offsets.